In a conventional radio communication system having power-saving control, for example, in a wireless LAN system described in Japanese Patent Application laid open No. HEI9-162798 (FIGS. 14 to 20) and “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications”, ANS/IEEE Std 802.11, 1999 Edition, pp. 128-133, the power-saving operation in a radio terminal is carried out by intermittently receiving beacons from a radio base station. That is, when a radio terminal enters power-save mode, the radio terminal obtains a beacon transmitted from a radio base station. Having extracted respective information elements included in the beacon, the radio terminal carries out intermittent receiving operation based on beacon interval information included within the beacon and an interval between beacons each having a delivery traffic indication map (DTIM).
When operating in the power-save mode, a radio terminal informs the radio base station of the operation using a frame control field. The radio base station buffers packets addressed to the radio terminal operating in the power-save mode in a memory within the radio base station, and informs the radio terminal that the packets have been buffered by a traffic indication map (TIM) included in a beacon.
The radio terminal operating in the power-save mode receives beacons intermittently from the radio base station. After receiving each beacon, the radio terminal extracts information elements. Having recognized from the TIM that the packets sent to the radio terminal itself have been buffered, the radio terminal sends the radio base station a control packet (PS-Poll) requesting the delivery the buffered packets, thus receiving its packets buffered by the radio base station.
Additionally, a broadcast/multicast packet is sent to the radio terminal subsequent to a beacon with the DTIM. The radio terminal receives at least a beacon with the DTIM so as to obtain the broadcast/multicast packet.
FIG. 1 is a sequence diagram schematically showing the operation of the conventional wireless LAN system in a continuous receiving mode for the normal operation and the power-save mode for the intermittent receiving operation. Radio terminals 1220 and 1230 belong to a radio base station 1210. The radio terminal 1220 is operating in the normal continuous receiving mode, while the radio terminal 1230 is operating in the power-save mode for carrying out the power-saving operation in a general wireless LAN system.
Since the radio terminal 1220 operating in the continuous receiving mode is always in the power-on or awake state, the radio base station 1210 transmits a packet (P20) without buffering it, the terminal 1220 can receive the packet (P20) without delay. On the other hand, the radio terminal 1230 operating in the power-save mode is controlled as to power-on/power-off in synchronism with the interval between DTIM beacons DB1 and DB2 transmitted from the radio base station 1210. After a packet P10 addressed to the radio terminal 1230 has arrived at the radio base station 1210, the radio terminal 1230 is informed by the next beacon with the DTIM that a packet sent to the terminal 1230 has been buffered. Having recognized from the information that the radio base station 1210 has buffered its packet, the terminal 1230 transmits a PS-Poll packet PS1 to the radio base station 1210, and receives its buffered packet P30 from the base station 1210 in response.
As just described, with the conventional radio communication system in a wireless LAN system, the power-saving operation in a radio terminal is conducted by intermittently receiving beacons from a radio base station. Besides, it is necessary to receive at least a beacon with the DTIM to obtain a broadcast/multicast packet.
Power saving on a radio terminal can be achieved by lengthening the interval between the receiving of beacons from a radio base station if there is no traffic. However, when the receiving interval is lengthened, the radio terminal can be late in obtaining the TIM, which causes a delay in receiving its packets.
In addition, a radio base station buffers packets addressed to a radio terminal operating in the power-save mode in its memory. Consequently, when the receiving interval is lengthened on the radio terminal operating in the power-save mode, delivery of packets to the radio terminal is delayed. Thus, the radio base station has to retain the packets in the memory.
In the real-time communication of voice, moving images, etc., if a radio terminal in the power-save mode operates at long receiving intervals when repeating transmission and reception, packets addressed to the radio terminal are once buffered by a radio base station and delivered thereto in the next receiving period. Accordingly, a packet delay occurs. In the real-time communication of voice and moving images, the delay in receiving packets on the radio terminal is a significant problem because this disrupts the reproducibility of data.
In the wireless LAN communication, the CSMA/CA (Carrier Sense Multiple Access protocol with Collision Avoidance) procedure is performed in order to avoid a collision during data transmission. Regardless of communicating application, the same IFS (Inter Frame Space) is applied to every control packet (PS-Poll) which a radio terminal transmits for requesting a radio base station to send packets when the radio terminal is operating in the power-save mode. Therefore, it is impossible to minimize delays and to give transmission right preferentially to the communication that requires real-time processing such as voice communication.
Moreover, the back off algorithm is used when data are to be transmitted to a radio terminal. That is, data are actually transmitted when random wait time has passed after the transmission right was given. Consequently, it is required to wait the random time regardless of the contents of transmission data, which makes it impossible to minimize delays in real-time communication such as voice communication.